The present invention relates to a semiconductor integrated circuit device and a process for fabricating the same and more particularly a bipolar semiconductor device having excellent high-frequency response and a process for fabricating the same.
As compared with the MOS semiconductor devices, the bipolar semiconductor devices are faster in operation but have the problems that the integration density is low and the power consumption is high. Without solving these problems, a further development of bipolar semiconductor devices would be impossible.
In the earlier stage of MOS semiconductor devices, there were the advantages in that the isolation between elements is not needed because MOS elements utilizes the inversion layers or channels in a wafer and that, as is clear from the silicon- or molybdenum-gate elements, the self-alignments can be utilized in forming the drains, sources and channels because the gate can be used as a mask for forming the source and drain. However, as the integration density is increased, the isolation between the elements; that is, the isolation of the drain and source of one element from those of adjacent elements are required. Thus the isolation problem must be solved for a further development of MOS semiconductor devices.
Meanwhile, in the bipolar semiconductor devices in the early stage, the epitaxial layers were very thin and of the order of 10 .mu.m and very wide isolation spaces were available for isolation. Since then the isolation technology has made a great advance now, the most thin epitaxial layer or film is two .mu.m in thickness. In addition, a wide isolation distance is not needed and the base and collector regions can be formed so as to be in contact with the isolation regions. Thus as far as the isolation problem is concerned, no distinct difference exists between the processes for fabricating the MOS and bipolar semiconductor devices. However, in the fabrication of bipolar semiconductor devices, a technique for self-aligning bases, emitters and collectors has not been available. As a result, the degree of shrinkage of bipolar semiconductor devices is by far lower than that of MOS semiconductor devices.
Another problem is encountered in the fabrication of bipolar semiconductor devices. In order to lower the base resistance, the graft bases (highly doped bases) must be formed as an external base region. When the impurities between the graft and active base regions are high in concentration and if the emitter and graft base regions are made into contact with each other, the degradation of breakdown voltage, burst noise and leakage current will result.
Thus as compared with the three regions of drain, channel and source of MOS elements, a bipolar element has four regions of emitter, collector, base and graft base so that the space for each element is greater.